Republicans need to pick up four more seats to take control of the Senate, and a year ago they had many plans for how to do so — none of which envisioned a battle to hold on to Indiana.

But Tuesday’s landslide victory in the GOP primary by Indiana state Treasurer Richard Mourdock, a staunch conservative who beat longtime Sen. Richard G. Lugar, gave Democrats hope for claiming a seat they have not seriously contested in three decades.

The sudden opening reflects a growing sense that the potential for big Republican gains has begun to ebb and that Democrats have a real chance of hanging on to their majority.

“Eight months ago, I thought that Republicans had a 60 to 65 percent chance of taking the majority. Now, it’s a 50-50 proposition as to whether Republicans can take the majority,” said Jennifer Duffy, a longtime expert on Senate races who works for the independent Cook Political Report.

Stuart Rothenberg, editor of the Rothenberg Political Report, said he places his “pinkie on the scale” now for Democrats retaining the majority, but added that his calculation hinges on economic improvements, particularly as reflected in the monthly unemployment numbers. “A few more months of less than 200,000 new jobs, and I take my pinkie off that scale,” Rothenberg said.

If Mourdock — a longtime politician twice elected to statewide office — can unify Republicans, he should be a favorite in GOP-leaning Indiana. But if his candidacy gets swept up in the fervor of the tea party movement, as some 2010 Republican nominees did, then Indiana could turn into a headache for national Republicans who would prefer not to expend resources to defend that seat.

“Lugar’s loss in Indiana put the seat in play, but only marginally improved Democrats’ chances of picking it up,” Duffy said.

But it was not supposed to come down to this: Republicans gained six Democratic seats in the 2010 midterm elections and, early on, 2012 looked to be even easier, with 23 Democrats up for reelection, compared with 10 Republicans. The Democratic president was deeply unpopular, and six Democratic incumbents and one independent, who generally votes with them, announced plans to retire. Those departures gave Republicans seven open-seat targets to pick from.

Democratic retirements in solidly red Nebraska (Ben Nelson) and North Dakota (Kent Conrad) seemed like sure GOP pickups, while vulnerable incumbents in Missouri and Montana, both of which Obama lost in 2008, seemed to be laying the groundwork for a GOP majority.

Originally, strategists and analysts predicted that this scenario would decide the majority: GOP gains in Missouri, Nebraska and North Dakota would push Republicans to 50 seats, and a victory in either Montana or Virginia would seal the elevation of Minority Leader Mitch McConnell (Ky.) to majority leader.

Republicans still appear to be in fairly good shape in Nebraska, even though Democrats recruited former senator Bob Kerrey to run. In Missouri, Sen. Claire McCaskill, who won with 50 percent of the vote in 2006, remains in a difficult spot, as polls show her in the low 40s, but Democrats say that the lackluster field of challengers in the August primary gives her a potential opening for victory. In North Dakota, Democrats have recruited former attorney general Heidi Heitkamp to face the winner of a June 12 GOP primary between Rep. Rick Berg and 2000 Senate nominee Duane Sand.

But the professional handicappers say that although they still expect GOP gains, Democrats are slightly favored to retain their majority, and the majority party is likely to hold just 51 seats — or 50, with the vice president serving as the tiebreaker.

According to Rothenberg and Duffy, Republicans have some lackluster candidates, particularly in three Democratic seats that were expected to be easier targets: Missouri, Nebraska and North Dakota. Republicans continue to be hopeful about those three seats in large part because each of those states has grown increasingly Republican in recent years, putting the Democrats at some disadvantage.

A few key primaries will determine the slate of candidates, particularly in Nebraska and Wisconsin, where tea party activists are trying to rally opposition to GOP establishment figures.

Their strongest recruits are Rep. Denny Rehberg (Mont.), whose district comprises the entire state and is a well-known figure, and former senator George Allen (Va.), who nearly won reelection in 2006 despite running a poor campaign. Those two will face off against well-known figures, too: Sen. Jon Tester (D-Mont.) and former governor Timothy M. Kaine (Va.), making those two races potentially epic battles that could go down to the final days of the campaign as neck and neck.

Even if Democrats lose most of those seats, the biggest shift since last summer has been the emergence of opportunities for Democrats to pick up Republican-held seats in Maine, Massachusetts and Nevada. These would mitigate the losses on their turf, setting up the possibility of only the second 50-50 Senate in U.S. history.

Former Obama adviser and Harvard Law professor Elizabeth Warren has emerged as a liberal icon and her Senate race against incumbent Scott Brown (R-Mass.) has become a rallying point for Democrats far beyond Massachusetts. Warren has raised nearly $16 million since she entered the race eight months ago. Her missteps over her claims of Native American heritage have raised questions about how she will handle the day-to-day work of political glad-handing that Massachusetts voters are accustomed to, but her campaign will have abundant resources in a state in which Brown will have to outperform presumed Republican presidential nominee Mitt Romney by roughly 500,000 votes in order to win.

In Nevada, Sen. Dean Heller (R) and Rep. Shelley Berkley (D) are in a tight race that could depend on how well Obama and Romney do in that battleground state.

The biggest break for Democrats came with the surprise retirement of Sen. Olympia J. Snowe (R-Maine), who, despite her moderate views, faced no real challenge in her primary and was likely to cruise to reelection. Now, former governor Angus King, an independent, is running for her seat and most observers think he will win and caucus with Democrats.