WASHINGTON » In a chamber of ample egos and matching oratory, Sen. Bob Casey, D-Pa., is understatement defined, his excitement most often registered by some extra eyebrow activity.

His low-radar personality, combined with exceedingly high unemployment and a dim view of President Barack Obama back in his home state, makes Casey — like so many Democrats in the Senate up for re-election next year — a perfect target for Republicans.

But like a number of his fellow Democrats whose fates next November could determine whether their party loses control of the Senate, Casey’s best asset may be his competition — in his case, a field of nearly a dozen Republicans, many of them unknown or divisive within their party.

Among his most prominent potential Republican challengers is Steve Welch, a deep-pocketed B entrepreneur from Chester County who in 2008 switched his registration to Democratic in protest against his own party and voted for Obama in the primary that year.

In a number of states where Republicans have been hopeful of picking up a seat, they are being hampered by some of the same dynamics that vexed their party in 2008, including trouble recruiting strong and experienced candidates, intraparty fighting, weak fundraising and the very same anti-incumbent sentiment that threatens Democrats.

Winning the Senate is tantalizingly within reach for Republicans, who have just 10 seats up for re-election, compared with the 23 that Democrats will defend next year, many of them in states where Democrats barely won in strong years for their party. National trends continue to favor Republicans, especially the weak economy.

But on a state-by-state basis, there are factors that give the Democrats hope and the Republicans pause. In states like Nebraska, tensions between the Tea Party movement and so-called establishment factions of the party threaten to roil the base. Florida and Pennsylvania have muddled Republican primary fields, with multiple candidates and no real standouts.

In Michigan, where Republicans hope the ailing economy is its own case against Sen. Debbie Stabenow, one Republican primary candidate, former Rep. Peter Hoekstra, proved to be an underwhelming statewide candidate in a previous campaign for governor, and another, Clark Durant, is an untested charter school executive.

The story is similar in Ohio, where Josh Mandel, the state treasurer, a Marine Corps veteran and fundraising powerhouse, is somewhat disadvantaged in his race against the incumbent Democrat, Sen. Sherrod Brown, by the fact the he looks too young to shave, several party officials acknowledged.

In Montana, Sen. Jon Tester, the Democratic incumbent, is giving the Republican candidate, Rep. Denny Rehberg, a spirited fight. Even in Missouri, where Sen. Claire McCaskill is the Democratic equivalent of the state’s flathead catfish — perpetually in open season — a group of Republican challengers have faced a series of setbacks, including fundraising problems, and the state’s most popular Republicans declined to jump in the pool.

“WhatB we learned from 2010 is primaries matter,” saidB Nathan L. Gonzales, an editor atB The Rothenberg Political Report, a nonpartisan newsletter. “Who Republicans nominate can dramatically affect the outcome of an election. Harry Reid was the best example of this; people didn’t like him, didn’t like the job he was doing, didn’t like the direction of the country, and they elected him because they didn’t like the alternative.”

Of 33 Senate races next year, an analysis by The New York Times finds that 15 of them are currently competitive. The analysis, which is based on an evaluation of the candidates, polling and fundraising results and the political and economic trends in the states, rates eight of them — Hawaii, Missouri, Montana, Nebraska, Virginia, Wisconsin, Massachusetts and Nevada — as tossups. Florida, Ohio, Michigan, Pennsylvania and New Mexico are rated as lean Democratic, while Arizona and North Dakota are rated lean Republican.

On balance, Republicans enter the 2012 Senate races with a solid chance of reversing the 53-47 advantage held by Democrats, which includes two independents who caucus with Democrats.

Democrats are largely playing defense, except in Massachusetts, where Elizabeth Warren is raising considerable amounts of money in the Democratic effort to unseat the original Tea Party upstart, Sen. Scott P. Brown, and in Nevada, where each party’s candidate is a member of the House but where voter registration favors the Democrat.

But Republicans have some concerns even in states that heavily favor them, including North Dakota, and Democrats are investing heavily in places like Nebraska, even though Sen. Ben Nelson’s voting record often has Reid, the Democratic majority leader, pulling at his gray tufts of hair.

The biggest fear among Republicans is of divisive primaries in which Tea Party-backed candidates prevail in states where they cannot win the general election, as happened in 2010 in Delaware, Colorado and Nevada, or weaken the preferred candidate in the process.

In Nebraska, Sen. Jim DeMint, the South Carolina Republican who is influential among grassroots conservatives nationwide, has endorsed Don Stenberg in the race to unseat Nelson, a Democrat. Stenberg is a conservative underdog whose fundraising has badly trailed that of Attorney GeneralB Jon Bruning, the frontrunner in that primary.

In 2010, DeMint similarly promoted the insurgent conservative candidacy in Colorado of Ken Buck, helping propel Buck to the nomination but allowing the Democrat, Sen. Michael Bennet, to argue that Buck was too far right.

Fears of ideologically divisive primaries often keep the best candidates from running, some Republican officials said.

“WeB are having trouble recruiting,” saidB Martha Breene, the chairwoman of the Venango County Republican party in Pennsylvania. “You often are not getting what you hope you could be getting, and then there is the Tea Party factor. A lot of them have good intent but it is sort of like they are the policemen of all things and they aren’t going to let other Republicans matter.”

In Pennsylvania, even Welch is trying to attract grassroots Republican activists to his campaign.

“I have a background that is similar to a lot of people in the Tea Party,” he said. “Historically I was not very engaged in politics, and I am a limited government person.” (Welch co-founded a company that assists developing businesses and briefly ran for the House in 2010.)

It was a scene that Rep. Patrick Meehan, a Republican who party leaders had hoped would enter the race, has decided he can live without.

“Sometimes the challenges are ones of the moment,” he said, explaining that he needed to serve his district. “We’ll see what emerges. It’s still not too late for anyone else to get in.”