A major subplot of this year’s midterm elections is the competition between the Republican establishment and the GOP’s tea party wing. The establishment is fighting back, but has the tea party already won?

The general election is still six months off. But Tuesday will open the summer preseason of intraparty contests, starting with an important primary in North Carolina. Between now and the end of June, more than two dozen states will hold primary elections. After a July break, the preseason will end with another round of primaries in August and early September.

The five primaries in which senators face direct challenges from tea party conservatives will be most closely watched for clues about the balance of power in the GOP. They involve Senate Minority Leader Mitch McConnell (Ky.), Thad Cochran (Miss.), Lindsey O. Graham (S.C.), Pat Roberts (Kan.) and Lamar Alexander (Tenn.).

All of those incumbents are favored to win. McConnell is first up, with a primary on May 20 against Matt Bevin. One earlier primary also involved a tea party challenge. That was in Texas, where Sen. John Cornyn easily survived.

What does this say about the tea party movement? For one thing, it may not be so easy to defeat incumbents in primaries. The reality is that senators (as well as House members) do not lose primaries often. They have lost them less often over the past three decades than in the 34 years before that.

Rhodes Cook, an independent elections analyst, laid out the statistics in a recent newsletter. He found that fewer members of the Senate and the House were defeated in primary elections between 1982 and 2012 than in the period from 1946 to 1980. In that first period, 38 senators and 147 House members lost primaries; since 1982, only eight senators and 74 House members have been defeated in primaries.

In House races, the most losses by incumbents came in the three post-redistricting elections — 1992, 2002 and 2012 — when incumbents were sometimes pitted against one another because of new district lines.

In 2010, three senators were denied their party’s nominations: Lisa Murkowski (Alaska), who went on to win reelection as a write-in candidate; Robert Bennett (Utah), who was knocked out in a party convention; and the late Arlen Specter, who lost the Democratic Senate primary in Pennsylvania after switching parties. But in no other year between 1982 and 2012 has more than one senator been defeated in a primary.

That provides a baseline for evaluating the tea party challenges this year. There should have been no grand expectations of incumbents falling left and right to the energized tea party wing of the GOP. But that is not necessarily because the movement has become significantly diminished.

Another reason the challengers might fall to the incumbents is the quality of the candidates. What the Republicans learned in 2010, in primaries involving their own incumbents as well as in primaries picking candidates to challenge Democrats, is that the tea party candidates often were not ready for the prime time of a general election.

Who can forget Nevada’s Sharron Angle or Delaware’s Christine “I am not a witch” O’Donnell? Both were weak general-election candidates and lost. In 2012, then-Sen. Richard Lugar (Ind.) lost his primary to conservative Richard Mourdock, who imploded once he became the nominee.

In 2010, Ken Buck, who had tea party backing, proved not quite ready for a general election and lost the Senate race in Colorado. He tried again this year, but Republican Rep. Cory Gardner engineered him out. The result was that, with Gardner in the mix, the Colorado race almost instantly became a toss-up, and Sen. Mark Udall is one more vulnerable Democratic incumbent.

Some tea party candidates have emerged as genuine rising stars within the GOP, Sens. Ted Cruz (Tex.) and Marco Rubio (Fla.) among them, although they have followed different paths in office. Sen. Mike Lee (Utah), who knocked off Bennett, has teamed with Cruz to make endorsements this year in GOP primaries in Nebraska and Oklahoma.

Another big reason the establishment-vs.-tea-party theme may be overdrawn is the degree to which the establishment has adapted to the new world by embracing tea party ideas and sometimes the movement’s confrontational posture. Slate’s David Weigel noted this in a piece about North Carolina recently. He observed that the establishment candidate in the GOP primary, state House Speaker Thom Tillis, has one of the most conservative legislative records in the country.

North Carolina Republicans will pick a challenger to Sen. Kay Hagan (D) on Tuesday, and one of Tillis’s opponents, Greg Brannon, is considered the tea party candidate. Brannon has the endorsement of Sen. Rand Paul (R-Ky.), who was in the state Monday to campaign for him. Tillis needs to win 40 percent of the vote on Tuesday to avoid a runoff. But he is an example of how the line between establishment and tea party has blurred.

On the basis of substance, the Republican establishment has drawn few boundaries between itself and the tea party. They may differ on tactics and procedures at times, and party leaders have disparaged some of the outside groups that have backed tea party challengers in primaries. But on the issues, the GOP is quite united and more conservative than it was a decade ago.

The real test of the movement’s influence is likely to come in the 2016 Republican presidential primaries. Adviser David Axelrod has said he told President Obama immediately after the 2010 midterm shellacking that there was one silver lining in what happened to the Democrats that year: He predicted that the results would push all the GOP presidential candidates, including Mitt Romney, further to the right in their bid to win the party’s nomination.

What will happen in 2016? The answer will depend in part on how Republicans do this fall and then on how the GOP presidential contenders read those returns. Will any of the viable Republican candidates try to put distance between themselves and the tea party, or will they generally embrace the movement? That question will be answered individually and collectively, and at that point, the tea party’s real standing will become clear.