It’s just on a month since we all stopped paying attention to the American primaries, when Rick Santorum conceded defeat and Mitt Romney became without doubt the presumptive Republican nominee.

But the primaries keep happening — the last, Utah, is not until June 26 — and this week’s were quite interesting. There was, for example, the fact that a Texas prison inmate won 41% of the vote against Barack Obama in the West Virginia Democrat primary, although no doubt this says more about West Virginia than it does about Obama.

There’s also the rather interesting fact that a lot of Republicans keep voting against Romney, even though he no longer has any serious opposition (Ron Paul has yet to officially withdraw). According to the figures at RealClearPolitics, Romney still only has a little over 43% of the popular vote in the primaries to date, even though he has well over 60% of the delegates.

Santorum and Newt Gingrich won almost 20% of the vote between them in Tuesday’s primaries, even though neither is campaigning, with another 13% to Paul.

But what really made the headlines was the Republican Senate primary in Indiana, where veteran senator Richard Lugar, first elected in 1976, was comfortably defeated by a hard-line challenger, Richard Mourdock, who was backed by the “Tea Party”.

That’s good news for the Democrats, since control of the Senate is very much in issue this year. (Because senators have six-year terms, the Democrats are defending their gains of 2006, which was a good year for them.) Lugar would have been very difficult to beat, but Mourdock will be a much easier target.

Still, that’s ultimately just one Senate seat. What’s more interesting is what it says about the state of the Republican Party and how that will play out in the presidential race.

There were multiple reasons to vote against Lugar: he’s 80, hasn’t lived in the state for years and was widely seen as out of touch. One could reasonably conclude that it was time for a change.

But the reasons that he incurred the Tea Party’s ire are very revealing.

He had voted to confirm Obama’s two selections for the Supreme Court. He supported citizenship for some illegal immigrants. And he won particular fame as a supporter of arms control, working with Democrats to establish a program to secure and dismantle nuclear weapons in the former Soviet Union.

For the Tea Partiers, it seems that the very idea of co-operation with the Democrats is unacceptable. That’s certainly Lugar’s take on his defeat: he accused Mourdock of promising “reflexive votes for a rejectionist orthodoxy and rigid opposition to the actions and proposals of the other party”. And that poses real problems for the way American politics is supposed to work; Jon Chait says it may contain “the frightening outlines of a future systemic crisis.”

It also shows, despite the volumes of commentary to the contrary, that the Tea Partiers don’t care much about economics. Their issues are cultural, driven by anger at the idea that women, gays, blacks and foreigners are taking over what they regard as “their” country.

And that in turn is the essential background to Obama’s offensive this week on same-s-x marriage. He knows that the issues that most energise the Republican base, which Romney has had to play to to secure the nomination, don’t go down so well with swinging voters. So he seems to have decided that he has more to gain than to lose by giving social issues a more prominent place in the campaign.

That’s not going to help him in West Virginia, but if he pushes the GOP further to the right it might win him support where he really needs it.