VALPARAISO, Ind. — Veteran Sen. Richard Lugar's loss in the Indiana GOP primary provides warnings for President Barack Obama and his Democrats as well as Mitt Romney and fellow Republicans six months before the November election.

In one state at least, anti-incumbent sentiment is coursing through the electorate, a potentially ominous sign for the incumbent Democratic president seeking a second term and lawmakers of all political stripes. The GOP also remains deeply split between the establishment wing and insurgent tea party, a fissure that underscores the challenge the presumptive Republican presidential nominee and other GOP candidates face in the months ahead to unite the party.

"We are experiencing deep political divisions in our society right now," Lugar, 80, one of the nation's longest-serving senators, said in a statement after the results were known. "These divisions have stalemated progress in critical areas. But these divisions are not insurmountable."

The loss of Lugar — who boasted of strong conservative credentials but was lambasted by critics for working with Democrats — also highlights the degree to which deal-makers are becoming a rarity on a Capitol Hill often consumed by partisan gridlock. He follows Sen. Olympia Snowe, R-Maine, a moderate known for bipartisanship, in leaving the Senate at year's end. Others too, including former Sen. Evan Bayh, D-Ind., have left in recent years.

Ultimately, it was Lugar's efforts to cross party lines and his longevity in Washington — two issues that tea party-backed challenger Richard Mourdock used against him — that proved too much for Indiana Republicans.

"Sen. Lugar has sided too many times with the Democrats," Stacy Rutkowski of Valparaiso, who voted for Mourdock, said on her way out of her polling place. "He's been there six terms, and it's time for some new blood."

Broadly, Lugar's defeat may create an opportunity for Democrats working to hang onto a narrow four-seat majority in the Senate. National party leaders vowed to help centrist Democrat Joe Donnelly, a three-term House member from South Bend, compete against Mourdock, the conservative state treasurer, in a Senate race the party otherwise would have bypassed.

But whether Democrats follow through with that pledge — and go all in for Donnelly by spending large sums of money in the race — is an open question. Indiana has been a hard place for Democrats to win. Four years ago, Obama became the first Democrat to carry the state in a presidential election since 1964, and he did so by a single percentage point, turning out vast numbers from the Chicago-influenced urban and industrial region in Indiana's northwest.

Democratic Senatorial Campaign Committee spokesman Matt Canter said the race, with Lugar out, "could move in a more competitive direction." The group and a political action committee that supports Democratic candidates began attacking Mourdock even before the polls were closed. Both described him as out of step with not just mainstream Republicans, but mainstream voters.

Democratic strategist Tad Devine said Mourdock's conservative profile has Democrats optimistic about their chances despite Indiana's Republican trend. Said Devine, "If the Senate race turns out to be a moderate Democrat and an out-of-step Republican, moderate voters who regret that they can't vote for Lugar will help Donnelly."

But Republican strategist Phil Musser doubts the state will be in play come the fall.

"Nationally, Democrats will throw a lot of money into it quickly. I'm not one who believes it will be a competitive Senate race," said Musser, a former Romney aide.

The race illustrated vulnerabilities for Democrats and Republicans alike.

Incumbents, Obama included, are at risk no matter their party at a time when the economically struggling public is sour over anyone linked to Washington. So, it seems, are lawmakers with a history of working with members of the opposite party.

Just ask Lugar.

Mourdock hounded the veteran senator over questions about his Virginia home — and Indiana residency — and his long Washington ties. The challenger also took Lugar to task over his collaboration with Obama. The two worked together on nonproliferation issues, and Lugar was one of only a handful of Republicans to vote to confirm Obama's two appointments to the Supreme Court.

It wasn't just those issues that didn't sit well with voters, who craved change after nearly four decades of Lugar representing them. That was clear from signs stating simply "retire Lugar" that dotted the roadside along U.S. 30 east of Valparaiso, a Republican-leaning town in northern Indiana.

"He's a good and decent man," Valparaiso Republican Bruce Garrison said of Lugar after casting his vote for Mourdock. "But how can the country keep going on the path it's on? And how can we send the same people back to fix it?"

It's that reject-the-status-quo strain among voters that incumbents up and down the ballot will find themselves having to fight against over the next six months.

That Lugar — an establishment candidate if there ever was one — fell to a tea party-backed Republican made clear that the divisions within the GOP that were on display in 2010 primaries across the country had not yet healed.

"There is an element of the Republican base, and it's stronger than ever now, that was never going to vote for Richard Lugar," said Dan Dumezich, a Lugar supporter from northwest Indiana and Romney's state co-chairman.

The split presents a huge challenge for Romney as he seeks to unify the Republican Party in the coming months.

He has campaigned as the establishment choice, but he was beaten badly at times by insurgent favorites, first former House Speaker Newt Gingrich in South Carolina and later in a series of contests by conservative former Sen. Rick Santorum.

Now Romney is working to mend the rifts. Whether he can — and whether the tea party and other conservatives rally behind him — won't be clear until November.