HELENA — Sen. Jon Tester and Rep. Denny Rehberg meet Monday night head-to-head in their first debate in more than three months.

When they last met in June, Tester, the Democratic incumbent, and his Republican challenger tangled over such issues as the Supreme Court's Citizens United campaign finance ruling, health care, regulation of the financial industry and other issues.

The Montana Senate is one of the most closely watched races in the country, one of a half-dozen or so pure toss-ups that will determine whether Democrats continue to control the Senate or Republicans take it over.

The Billings debate, televised on Montana PBS and C-SPAN, could get testy.

Rehberg and his Republican allies have been attacking Tester as an ally of President Barack Obama who hasn't lived up to his promises to be independent, such as by being a top recipient of campaign donations from lobbyists. The federal health care law has particularly been a drag on Tester.

Tester and other Democratic groups have been portraying Rehberg as a career politician with few results to show for it, and as a land developer who only poses as a rancher. They argue Rehberg has made questionable choices, such as suing his hometown's fire department for a wildfire that burned across his Billings land.

Rehberg has made it a postelection priority to undo the health care law, cut business regulation and reduce spending. Tester has said his top issue after re-election would be to resuscitate a bipartisan balanced budget solution that has languished in Congress.

Millions have been spent by both sides, saturating the state's small television markets with a steady stream of advertising. Polls show few voters are undecided as Rehberg holds a slim lead.

Rehberg announced earlier Monday that he had for the first time topped Tester in campaign fundraising in the most recent quarter.

Rehberg hauled in $2.4 million and held $1.7 million cash-on-hand with less than a month to Election Day. Tester brought in $2.3 during the quarter and held $1.3 million.

Both sides are also spending a great deal to rally their base to the polls.

With Obama almost certain to lose in the state, Tester is banking on the state's ticket-splitting tradition where it has often picked Democrats for lower offices while endorsing the Republican presidential candidate.