After a series of stringing defeats in the 2010 and 2012 Senate primaries, the GOP Establishment regrouped and won major intraparty victories, spending millions of dollars in primary races nationwide in 2014.

Both sides of the Republican world will be looking to key Senate primary contests to help them take the initiative in 2016. Here are a few of the big battles shaping up so far.

Florida — A Marquee Race to Replace Rubio
Florida’s marathon contest to replace one-term Sen. Marco Rubio could be the most significant of all the contests, serving as a microcosm of the battle nationwide between the Establishment and the base — on both sides of the aisle.

Desantis (left) and Jolly (right)
Rep. Ron Desantis (left) and Rep. David Jolly (right) are essentially tied in the GOP contest to replace Sen. Marco Rubio.

The deciding battleground for which wing of the Republican Party will emerge victorious in 2016 may well be the GOP must-win Sunshine State.

Two GOP congressmen are battling for front-runner status in the competition. While the race to date has largely been defined through preliminary skirmishes, the showdown has all the trappings of a marquee Establishment vs. Conservative brawl.

Front-runners Rep. David Jolly and Rep. Ron Desantis are running neck and neck, and both are significantly ahead of third place candidate Lt. Gov. Carlos López-Cantera.

Desantis notches a 90-percent grade from Heritage Action, the political-wing of the Heritage Foundation conservative think tank run by former Sen. Jim Demint. Jolly, on the other hand, pulls in a paltry 40 percent score from the conservative policy group.

But Jolly pulls a 100 percent 2014 rating from the U.S. Chamber of Commerce, while Desantis lags in the Establishment mega-group’s report card with a 64 percent rating.

The race is ripe for a battle royale come the spring between groups like the conservative Super PACs of Club for Growth, Freedom Works and the Senate Conservatives Fund and the Establishment-aligned Chamber.

Florida Democrats have their own ideological battle looming.

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Progressive firebrand Rep. Alan Grayson is sure to look to socialist Sen. Bernie Sanders, should he continue to run strong in the primaries, to build a case against Democrat-Establishment favorite Rep. Patrick Murphy.

Democrats eyeing a comeback to power in the Senate need to pick up five net seats to retake the upper chamber, and all signs point to Florida as a critical piece of the puzzle.

Senate Democrats are already salivating over their prospects in blue states Illinois, Wisconsin and New Hampshire and are looking to Florida, Ohio and Pennsylvania to press the field, and possibly retake the Senate come the fall.

The Washington, D.C., Democrats have coalesced around Murphy as their candidate to win the gold. But Sanders has proven on the national stage an insurgent left-wing candidate can wage a successful, prolonged battle against a Democrat Establishment favorite, and Grayson has personal wealth he can call on to offset Murphy’s massive fundraising advantage.

Of the three second-tier states Democrats appear poised to target, only one is rated “Tossup” by the veritable Cook Political Report — Florida.

The strength of the eventual nominees from each party will inevitably impact the outcome of the crucial presidential contest in the Sunshine State, infamous for razor-thin margins.

Arizona — John McCain Faces a Determined Challenge
The blustery, and obstinate former GOP presidential nominee faces perhaps his strongest primary challenger yet. After successfully fighting off a challenge in 2010 by 24 percentage points, McCain, nevertheless, continued his maverick tendencies, which have long infuriated conservatives.

Kelli Ward (left) and John McCain (right)
State Sen. Kelli Ward (left) is fashioning herself the first intra-party challenger who can defeat Sen. John McCain (right)

The Arizona senator hammered Ted Cruz in the media, calling the Texas senator a “wacky bird.” Poking his finger into the eye of conservatives on immigration, McCain joined Rubio, Sen. Chuck Schumer, D-N.Y., and others in the Gang of Eight 2013 amnesty push, which was ultimately defeated by intense conservative opposition.

McCain’s challenger is state Sen. Kelli Ward. Ward is largely untested in a high-profile election but appears polished and willing to put in the legwork to rally disparate conservative groups to her cause.

In a sign of grassroots potential for Ward, the GOP Committee of the state’s largest county voted recently to support any candidate other than McCain in the Republican primary.

McCain, however, is a battle hardened campaigner sitting on top of a $5-million war chest. By the end of 2015, Ward had raised $500,000 — a respectable, but not yet competitive amount.

McCain will have to run a real race to defend himself, but will likely survive unless Ward can rally big spending national conservative groups to her cause and make the contest more even.

Indiana — A Sleeper Insurgent vs. the Establishment’s Man
In the first full fundraising quarter with both front-runners formally in the race, Establishment-preferred Rep. Todd Young brought in $2.2 million. That was double the haul of more conservative Rep. Marlin Stutzman, who brought in just over $1 million. But Strutzman has earned the open endorsement of the well-funded and formidable Club for Growth.

Todd Young (left) and Marlin Strutzman (right)
Rep. Todd Young (left) leads Rep. Marlin Strutzman (right) in fundraising, but the race is only just developing.

But to Young’s advantage, the conservatism gap is less pronounced in the Indiana race than in the Florida competition. Young notches a 61 percent score from Heritage Action and Struztman 87 percent.

Young will have to play up his acceptability with conservatives, particularly since retiring incumbent Sen. Dan Coats’ Chief of Staff Eric Holcomb is running and will be looking to carve out a constituency among more moderate, Establishment-friendly voters.

There has been scant public polling on the race to date, but the contest is likely to line up with Young the front-runner and Struztman, his chief adversary. Strutzman will need to lay out a compelling case to conservatives to support him over Young and will need the Club for Growth and others to invest resources to overcome his financial disadvantage.

Strutzman was set back from building his case late last year when much of his senior staff abandoned the campaign. It remains to be seen whether his new operation can build the infrastructure and narrative to seriously challenge Young and invite the outside spending that could put him over the top.

Alaska — Will Murkowski Go Untouched?
No serious challenger has emerged yet to test the conservative-loathed Lisa Murkowski. But the Alaska senator, who lost a GOP primary in 2010 only to fight back and win a write-in campaign, can never be struck from a list of potentially endangered incumbents.

Sen. Lisa Murkowski
Sen. Lisa Murkowski has yet to draw a serious GOP primary challenger for 2016.

There are several intraparty challengers who could make a serious bid against Murkowski, though time is running out.

The 2010 GOP candidate who bested Murkowski, radio host Joe Miller, said in a Jan. 5 interview with the AP he is not seriously considering another run at her, knocking out of contention her most natural possible opponent.