The Republican party’s march ever further to the right continues, as seen in the defeat yesterday of one of the Senate’s true veterans, Richard Lugar, Republican from Indiana, in the party primaries by Tea Party favorite Richard Mourdock, the State’s treasurer.

Lugar’s loss could pave the way for a Democratic victory in the November election. We’ve seen this happen before, as in Nevada and Delaware in 2010, when the Democratic candidates Harry Reid and Christopher Coons both clearly defeated their Tea Party opponents. A victory this fall for the Democratic candidate Joe Donnelly in a traditional Republican stronghold such as Indiana would be a big win the Democrats in the fierce battle over which party wins the majority in the next Senate.

Richard Lugar, Senator since 1976, foreign policy expert, one of the last of the old school of moderate Republican senators, was bitter, and worried, after his defeat:

“If Mr. Mourdock is elected, I want him to be a good Senator. But that will require him to revise his stated goal of bringing more partisanship to Washington. He and I share many positions, but his embrace of an unrelenting partisan mindset is irreconcilable with my philosophy of governance and my experience of what brings results for Hoosiers in the Senate. In effect, what he has promised in this campaign is reflexive votes for a rejectionist orthodoxy and rigid opposition to the actions and proposals of the other party. His answer to the inevitable roadblocks he will encounter in Congress is merely to campaign for more Republicans who embrace the same partisan outlook. He has pledged his support to groups whose prime mission is to cleanse the Republican party of those who stray from orthodoxy as they see it. This is not conducive to problem solving and governance. And he will find that unless he modifies his approach, he will achieve little as a legislator.”

As Nate Silver points out on his blog FiveThirtyEight, Richard Lugar is the latest in a series of moderate Republican Senators to be defeated. Of the 27 moderate Republicans in the Senate just a few years ago, only six will remain after the November elections. In contrast, only 11 of the 28 conservative Republicans will be gone next year.

“The Republican Senate is starting to grow as conservative as the Republican House,” writes Silver.