The tea party referendum has officially begun.

Sen. Dick Lugar (R-Ind.) faces a tough primary challenge. (J. Scott Applewhite/AP Photo)

But that doesn’t mean the tea party doesn’t have its chances this year.

The Club for Growth’s endorsement of Indiana state Treasurer Richard Mourdock over Sen. Richard Lugar (R-Ind.) this week cements that race as the biggest tea party-versus-establishment Senate contest of the cycle. And it may set the tone for the rest of the year.

But there are six other tea party-versus-establishment races worth keeping an eye on. The Fix looks at each of them in chronological order.

* April 21: Utah state nominating convention: This is where Sen. Robert Bennett (R-Utah) was unseated two years ago, and now tea party types are eyeing Sen. Orrin Hatch (R-Utah). Hatch’s top opponent is state Sen. Dan Liljenquist. Either man secures the nomination if he gets 60 percent of the vote at the convention; otherwise they go to a one-on-one primary.

* May 5 — Nebraska primary: State Attorney General Jon Bruning has been endorsed by the Tea Party Express, but state treasurer Don Stenberg better fits the bill of tea party candidate; he’s meagerly funded, a conservative crusader and has the backing of Sen. Jim DeMint (R-S.C.).

* May 29 (?) — Texas primary: We don’t yet know when Texas’s primary will be, due to the current court battle over redistricting. But whenever it’s held, the tea party will be pulling for former state solicitor general Ted Cruz over the very well-funded Lt. Gov. David Dewhurst. For now, the name of the game for Cruz is at least finishing second and hoping for a runoff with Dewhurst.

* Aug. 7 — Michigan primary: Businessman Clark Durant is hoping to take down former congressman Pete Hoekstra and has secured some establishment support in that quest. Hoekstra has a big early lead, though, in large part because basically nobody knows who Durant is yet.

* Aug. 14 — Wisconsin primary: Former governor Tommy Thompson is looking like he may actually be an underdog against former congressman Mark Neumann, another Club for Growth-endorsed candidate. After Indiana, this may be the contest with the most tea party passion behind it.

* Aug. 28: — Arizona primary: As in Nebraska, this one doesn’t fit neatly into a tea-party-versus-non-tea-party box; this is more about outsider versus insider. Businessman Wil Cardon is the outsider candidate has self-funded $1.3 million so far to defeat Rep. Jeff Flake in the GOP primary. At the same time, Flake has strong ties to the tea party movement and has been endorsed by the tea party group FreedomWorks. As in Indiana, a loss by the establishment candidate here could put this seat very much in play for the Democrats, who like their chances with former U.S. surgeon general Richard Carmona.

If Cardon and Mourdock win, expect to see those two races on this list in the near future. In the meantime, only Wisconsin and Nebraska make the cut.

To the line!

10. Ohio (Democratic-controlled): The most recent Quinnipiac poll shows Sen. Sherrod Brown (D) growing a 48 percent to 35 percent lead over state treasurer Josh Mandel. Mandel has plenty of time and money to introduce himself to voters, and the presidential race all but ensures a tough fight here. But Mandel has yet to show he’s a top GOP hope. (Previous ranking: 8).

9. Wisconsin (D): Thompson is trying to make a political comeback more than a decade after leaving office as governor to become HHS secretary, and he’s still got to prove that he can run a 21st-century campaign. In his first quarter of fundraising, Thompson raised a pedestrian $650,000 — only slightly more than Neumann. The GOP acknowledges this primary will be hard-fought and that Rep. Tammy Baldwin (D) is raising good money. But it hopes that she’s too liberal for the state. (Previous ranking: 10)

8. New Mexico (D): Lt. Gov. John Sanchez’s decision to bow out of the GOP primary gives an all-but-clear path to the nomination for Wilson, who can save money and position herself as a moderate for the general election. Rep. Martin Heinrich (D) still has a primary from state auditor Hector Balderas, but he has been outraising Wilson and still would probably have a slight edge in this nominally Democratic-leaning state (Previous ranking: 9).

7. Virginia (D): The U.S. Chamber of Commerce has been hitting former governor Tim Kaine (D) hard, tying him to President Obama. But with polls showing a continually deadlocked race between Kaine and former Virginia governor and U.S. senator George Allen (R), it’s unlikely much will change here until the fall. (Previous ranking: 7)

6. Nevada (Republican-controlled): Appointed Sen. Dean Heller (R) is showing some signs of (fundraising) life. After getting badly outraised for two straight quarters, he picked it up in the fourth quarter, matching Rep. Shelley Berkley’s (D) $1.1 million haul. The GOP probably needs to hold this seat if it wants to have any chance at winning the Sante majority. (Previous ranking: 5)

5. Montana (D): A bipartisan poll recently showed something we already knew — that the race between Sen. Jon Tester (D) and Rep. Denny Rehberg (R) will be close. The poll for Colorado College’s “State of the Rockies” series showed Rehberg up slightly, 46 percent to 43 percent. Much like Nevada is a bellwether for the GOP, Montana is probably the dividing line between Democrats potentially losing control of the Senate and keeping it. (Previous ranking: 6)

4. Missouri (D): A Republican primary crowded with weak fundraisers is helping Sen. Claire McCaskill (D). No strong challenger has emerged, though an endorsement from Sen. Mike Lee (R-Utah) might give Sarah Steelman a boost. Polls still show that Rep. Todd Akin, businessman John Brunner and Steelman could all give the vulnerable senator a tough race, and GOP outside groups are helping with the attack. With the field struggling, though, keep an eye on state Auditor Tom Schweich, who is now considering joining the race, according to a local blog.. (Previous ranking: 4)

3. Massachusetts (R): Just when we thought Elizabeth Warren (D) was on path to beat Sen. Scott Brown (R), a Suffolk University poll from this morning suggested it won’t be so easy. The poll put Brown on top by 9 percent, while previous polling had shown Warren tied or leading. We don’t know whether this as an outlier or a sign of a real shift, but Brown has to be happy that his campaign has — at least momentarily — stopped the slide. (Previous ranking: 3)

2. North Dakota (D): For the first time since Sen. Kent Conrad (D) announced his retirement, this race no longer tops our last. The reasons for this are threefold: First, former state attorney general Heidi Heitkamp looks to be a pretty legitimate Democratic nominee. Second, Rep. Rick Berg (R) isn’t terribly popular right now. And third — well, see below. (Previous ranking: 1)

1. Nebraska (D): With Sen. Ben Nelson’s decision to retire, this seat instantly became the one most likely to change hands — taking over from the long-time No. 1, North Dakota. Former senator Bob Kerrey’s decision to pass on another bid only made things worse for Democrats. Bruning, who didn’t start off great, can now afford to stumble a bit in the GOP primary. (Previous ranking: 2)

Rachel Weiner contributed to this report.