There are two ways to explain why some veterans of Congress find themselves in unexpectedly deep trouble come voting time.

One theory: political Darwinism.

Like natural selection and the thinning of the herd, the candidate is long in the tooth and a step slower – and slow to react to a younger, more cunning predator challenger.

Another theory: political climate change.

Like a shift in temperature, the electorate undergoes a shift in thinking – about Washington and the officeholder’s relevance. Not a thinning of the herd – more like an anti-incumbent, herd mentality.

Keep this mind if, a week from now, Republicans in Indiana kick Sen. Richard Lugar to the curb in the state’s May 8 primary.

If indeed Lugar is ousted in favor of his challenger, state treasurer Richard Mourdock, then yes natural selection will have played a significant role. Lugar, age 80, is the nation’s the third most senior U.S. senator (behind only Hawaii’s Daniel Inouye and Vermont’s Patrick Leahy).

More signs of rust: Lugar’s lived inside the D.C. beltway since entering the Senate in 1977 – his Indiana residency becoming a campaign controversy earlier this spring. Moreover, he hasn’t been pushed in a race in over three decades (in 1982, he was re-elected to a second term by a 8% margin; in 2006, he ran for a sixth term unopposed, collecting 87% of the vote).

So you can say that, if he loses, it was simply a matter of Lugar’s time running out. Then again, look at California, where Democratic Sen. Feinstein, a 20-year Senate veteran who turns 79 this summer, is expected to breeze to a November win.

The more compelling storyline, should Lugar lose: climate change – and what’s become a familiar tale of changing ideological fault lines for Republicans.

Examples:

In 2004, Pennsylvania Sen. Arlen Specter, a moderate with 30-plus years in Washington, finds himself in a death fight with conservative challenger Pat Toomey. Toomey attacks Specter as a fiscal wimp; Specter turns to his party’s establishment for a bailout – President Bush and then-Sen. Rick Santorum offering endorsements (a decision Santorum would later regret). Specter avoided the upset with 51% of the primary vote. However, six years later and facing a likely primary loss in a rematch with Toomey, he ditched the GOP – only to lose in the Democratic primary.

In 2010, third-term Utah Sen. Robert Bennett finished third in his state party’s nominating convention. Credit this one to Tea Party activists, who punished the 76-year-old Bennett for his 2008 vote in favor of the TARP bank bailout, plus his support of a bipartisan healthcare mandate (though he did vote against Obamacare).

In 2012, Utah’s Orrin Hatch – at 79, in search of a seventh term and trying to avoid Bennett’s fate in his state’s June 25 primary – has spent at least $5.7 million (that’s eye-popping, by Utah standards) to fend off his lighter-in-the-wallet challenger. Unlike Bennett, Hatch actually won at the nominating convention – but fell a few votes shy of the 60% needed to avoid a primary runoff. What has Hatch worried? He’s been targeted by the FreedomWorks super PAC, for a variety of Tea Party offenses (voting for the TARP, auto and Fannie and Freddie bailouts; voting to confirm Federal Reserve Chairman Ben Bernanke, Treasury Secretary Timothy Geitner and U.S. Attorney General Eric Holder).

Getting back to Indiana and Lugar, it’s easy to see his troubles as a continuation of this trend.

As with the aforementioned races, it’s a contest that’s turned into a referendum on the incumbent’s record (including Lugar’s support of bailouts, ethanol and a ban on assault rifles), with the suggestion that the longtime Washington insider parted ways with the conservative faith.

As was true in Bennett’s convention setback, Lugar was slow to realize just how much trouble he faced. As Indiana’s primary neared, Lugar went on the offense, trying to portray his challenger as “untrustworthy” and not suitably conservative – some have suggested, a tactical maneuver that came too late in the race.

And, as with that 2004 Republican primary in Pennsylvania (Specter vs. Toomey; the limited government Club for Growth vs. the Bush 43 White House), Lugar-Mourdock is a race that again pits GOP insiders vs. the outsider tea partiers (John McCain’s cut a radio ad for his Senate colleague; Sarah Palin’s endorsed Mourdock on the candidate’s Facebook page).

Will Lugar prevail in the May 8 primary? An early April survey had the contest at Lugar 42%, Mourdock 35%. Mourdock’s campaign is touting an internal poll showing the challenger with a narrow lead.

And should Lugar lose? It means that, come the morning of May 9, you’ll wake up to a lot of political chatter about a schism between Tea Party activists and establishment Republican – “breaking news” that in fact is old news in GOP campaign circles.

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