Sen. Richard Lugar (R-Ind.) won't be returning to the Senate next year, but he's not a forgotten man in the race to replace him. With 18 days left until Election Day, Lugar’s cold shoulder is complicating GOP candidate Richard Mourdock’s bid against Rep. Joe Donnelly (D).

(Michael Conroy -- Associated Press)

The latest development in an ongoing saga is a direct mail piece from a super PAC supporting Mourdock. The Indiana political news Web site Howey Politics Indiana reported Wednesday that USA Super PAC sent mail pieces indicating that “Indiana's Lugar Backs Mourdock in Senate Run." Lugar’s office said it didn’t authorize the ads.  “It was clearly unauthorized and done without consultation with us. Lugar clearly stated on September 17 that he would not campaign for Mourdock in the general election for Senator from Indiana,” Lugar spokesman Andy Fisher told Howey Politics.

The mail piece cites a September report in which Fisher said, “Yes, the senator is supporting Treasurer Mourdock.”

While it’s true that Lugar has expressed support for Mourdock, their relationship is hardly a warm one.

Mourdock dislodged Lugar in May a primary that wasn’t even close. Running to the longtime senator’s right, Mourdock won a heated campaign in which he routinely attacked the senator for being insufficiently conservative.

The defeat didn't appear to sit well with Lugar. In his concession speech, the senator expressed hope that Mourdock “prevails in November” in order to help Republicans take back the majority. But he never stumped for the his onetime opponent  And in September, he confirmed that he had no intention to campaign for the man who defeated him.

Lugar and Mourdock are very different Republicans. Mourdock’s unapologetic conservatism clashed with Lugar’s history of working with Democrats. Had Lugar won, the general election would hardly be as competitive as it stands now. The longtime senator’s ability to secure crossover support would have left Donnelly with very long odds.

That’s why Democrats were elated after Mourdock's primary victory. Not only did it mean Donnelly would face a more conservative Republican who would have a tougher time with independents and Democrats; they were also presented with an opportunity to seize on some of the bad blood in the primary and peel off some Republican support from discontented Lugar supporters. Even if those supporters didn’t vote for Donnelly, the thinking went, some might opt to stay home.

None of this would matter much if the race wasn’t so close. But it is. A bipartisan Howey/DePauw poll released late last month showed Mourdock and Donnelly running about even. In the poll, Mourdock had the support of 71 percent of Republican voters; and only about six-in-10 Lugar primary voters were backing Mourdock. In other words, hardly the command of the Republican base one would hope to have behind them in a close race. The latest flap over the pro-Mourdock mailer only threatens to fan the flames.

“If you are looking for an excuse as a Lugar voter to not like Mourdock and go for Donnelly instead, this certainly gives you an excuse,” said Ed Feigenbaum, author of the Indiana Legislative Insight newsletter.

"I know that there are hard feeling[s] that may not have healed from the primary but it is incumbent on Republicans to make sure that Congressman Donnelly is not allowed to cynically exploit the situation,” James Bopp, who heads USA Super PAC, said in a statement Wednesday.

Republicans remain confident that the race hinges much more heavily on the records of Donnelly and Mourdock. Indeed, Donnelly’s votes in support of some of President Obama’s signature measures could sink him in a state Mitt Romney is expected to win

And while one of Indiana's senators isn't stumping for Mourdock, the other one is. Sen. Dan Coats (R) cut a TV ad for Mourdock, which his campaign released this week. Both Romney and vice presidential running mate Paul Ryan (Wis.) have also campaigned for Mourdock. 

When asked how the Lugar-Mourdock dynamic would factor into the final three weeks of the campaign, Mourdock spokesman Christopher Conner said: “Richard appreciates Senator Lugar's call for Hoosiers to support his election and has often praised his exemplary public service.”

The Mourdock campaign turned to Republican Sen. John McCain earlier this week. The Arizonan cut an ad for Lugar during the primary, but was in the state to stump for Mourdock on Wednesday

This isn’t the first time the results of a Senate primary have triggered some post-primary discomfort. In 2010, Republican Mike Castle wouldn’t endorse tea party candidate Christine O’Donnell after a stinging defeat. Democrat Christopher Coons easily defeated O’Donnell that November. 

Another example is 2002, when Republican John E. Sununu defeated incumbent senator Robert Smith (R) in New Hampshire, resulting in icy relations leading up to November, when Sununu ended up winning by about four points.

The Lugar-Mourdock relationship (or lack thereof) probably won’t decide the outcome of the race on its own. Other factors like ads, messaging and fundraising (Mourdock nearly doubled up Donnelly in the third quarter) are the main drivers of the campaign. But if the margin stays close down the stretch, the senator about to leave office could leave a larger-than-expected imprint on the campaign to succeed him.