Here's where things stand in the battle for the Senate: Republicans' best pickup opportunities are in two open seat races and one where a recently appointed senator is running: West Virginia, South Dakota and Montana. Republicans are favored to win all three. From there, their four best remaining chances are in states where Mitt Romney won and Democratic incumbents are running for reelection: Arkansas, Louisiana, North Carolina and Alaska.
But Republicans are playing defense, too. Democrats have recruited strong candidates in Kentucky and Georgia. If they pick up even one of those seats, Republicans would have to run the table in the seven races above to win the majority, provided nothing else flips. If Democrats win both, Republicans must win those seven plus one from their next two best pickup opportunities: Michigan and Iowa.
Each scenario would be a very tall task that would leave the GOP with no margin for error.
This is where Brown comes in. He's now part of a group of Republican candidates that includes Cory Gardner and to a lesser extent, Ed Gillespie. New Hampshire instantly becomes more competitive by virtue of Brown's decision. Up until now, no other Republican with a prayer of defeating Sen. Jeanne Shaheen (D-N.H.) had entered the race. Brown's name recognition and his ability to raise big money make him a potentially formidable foe.
Something similar happened in Colorado when Gardner, a sitting member of Congress, announced last month that he would take on Sen. Mark Udall (D-Colo.). And while Sen. Mark Warner (D-Va.) is still a substantial frontrunner in Virginia, Gillespie, a former Republican National Committee chairman who announced his run in January, gives Republicans a glimmer of hope in Virginia that did not exist before he decided to run.
In short, Republicans now have more possible routes the the majority.
Even if Brown, Gillespie and Gardner each lose but wage competitive campaigns, they will force Democratic groups to spend money they wouldn't otherwise have to spend in these races. That's money that could go to races like Arkansas or North Carolina, for example.
Brown is no shoo-in. If he decides to turn his exploratory committee into a full-fledged campaign, he will face obstacles in a primary. His stance on guns -- which is a big issue for Republican voters in New Hampshire -- most notably his support for an assault weapons ban, could be a problem for him. So could his pro-abortion rights posture. Finally, the primary is not until September, giving his opponents -- and Democrats -- time to try to bloody him up.
Even if Brown makes it to the general election, he is probably an underdog to Shaheen. A recent Suffolk University poll showed him down 52 percent to 39 percent.
Candidates don't come out of the woodwork to run for U.S. Senate in a vacuum. The decisions of Brown, Gardner and Gillespie reflect a political climate that has increasingly given Democrats reason to worry. Polls show President Obama is unpopular; the health-care law remains a potential albatross on the campaign trail; and Democrats just lost a very winnable Florida special election to a Republican former lobbyist who struggled to raise money.
Republicans increasingly feel like they are poised to go on offense this year. On the Senate landscape, they certainly have reasons to be upbeat.