It is a little more than eight months until Election Day, but even now two Republican-held Senate seats look increasingly like the keys to whether Democrats can hold their narrow majority in November.

Republican strategists are overwhelmed with opportunities and potential opportunities this cycle, and even a modest breeze at the GOP’s back is likely to swing the Senate rather dramatically to it this fall.

But if the November elections take place during a relatively neutral partisan environment, or even one narrowly favoring the president’s party, Democrats may find that Republican-held seats in Massachusetts and Nevada determine whether Harry Reid remains the Senate Majority Leader in 2013.

With five Democratic seats at greatest risk — Nebraska, North Dakota, Missouri, Montana and Virginia — and another five definitely in play — New Mexico, Ohio, Florida, Wisconsin and Hawaii — the 2012 Senate landscape is stacked in the Republicans’ favor.

Nobody doubts that the GOP will pick up some Democratic seats. The only question is how many. And with the Republican presidential nominee likely to carry at least four (and possibly as many as nine) of the 10 states that include Republican Senate opportunities, it is easy to see Democrats losing four or five of the seats that they are defending in November.

It would be wrong to call Massachusetts and Nevada the Democrats’ firewall, but those two states, one reliably Democratic and the other competitive, are important to Democratic strategists in their efforts to retain a Senate majority.

If Democrats can win those two contests against Republicans who didn’t earn their way to the Senate quite the same way as their colleagues did, they will make it that much more difficult for the GOP to net the four seats that the party will need if President Barack Obama wins a second term.

Massachusetts is surely the best opportunity for a Democratic takeover of a Republican seat.

First-term Sen. Scott Brown is good-looking, personable and politically astute. He separates himself just enough from his party to make a credible case of being “independent” without causing Republican voters and interest groups to see him as another Arlen Specter.

Face it, if charisma guaranteed re-election, Brown could start picking the colors he wanted to repaint his Senate office.

But the Republican was elected in a special election against a Democratic opponent who turned out to be so inept that she would have made the 1962 Mets look like the 1927 Yankees.

This time, Brown will be running against an icon of the left, Elizabeth Warren.

Republicans like to caricature Warren as a liberal Ivy League college professor, but even putting aside her stunningly impressive fundraising numbers, she has had a solid campaign launch.

November’s electorate will be much larger and very different than the one that elected Brown in January 2010, when voters in the Bay State surely saw the special election as an opportunity to send a message to Washington and to Obama about their dissatisfaction with the direction of the country and the president’s performance.

The president will carry Massachusetts by a large margin (even if it isn’t as large as his almost 800,000-vote victory in 2008), so Brown will need to have many hundreds of thousands of Obama voters cross over and vote for him in the Senate race if he is to keep his seat in November.

That’s not impossible (particularly if Warren doesn’t sell over the long haul), but it is no easy trick, even for a savvy politician who has quickly built his own brand.

In Nevada, Republican Sen. Dean Heller will try to hold off Democratic Rep. Shelley Berkley in what looks to be another very competitive race. Unlike Brown, Heller is something of a squatter in his seat. Never elected to the Senate, he was appointed to fill the post by a Republican governor after then-Sen. John Ensign (R) resigned in disgrace.

At that time, Heller was representing the state’s 2nd district, which takes in Reno, Carson City and politically important Washoe County. Of course, Heller has run statewide, serving two terms as Nevada’s secretary of state.

Berkley, on the other hand, is a seven-term House Member who in some senses epitomizes Las Vegas, which she represents in Congress.

Berkley has an outgoing personality and is an aggressive campaigner. The Congresswoman’s style and liberal bent may well fit her Las Vegas district (and even somewhat the rest of Clark County), but it’s less likely to have strong appeal in the rest of the state.

Berkley’s persona, combined with Heller’s more easygoing style and the state’s competitiveness, gives Republicans a pretty good chance to hold the Silver State’s Senate seat — if Heller works hard to keep it.

If 2012 turns out to be a pretty good Republican year, the GOP could hold both the Massachusetts and Nevada seats and grab five to eight more Democratic-held seats, building a strong Senate majority.

But if 2012 turns out to be a pretty good year for Democrats (and Obama), Massachusetts and Nevada easily could go Democratic, and the president’s party could keep its net Senate losses down to a couple of seats, hanging on to control for a couple of years and setting up 2014 as an excellent “six-year itch election” opportunity for the GOP.

Keep an eye on Massachusetts and Nevada for clues on how the cycle is developing.

Stuart Rothenberg is editor of the Rothenberg Political Report.