Senate Appropriations Committee member Mary Landrieu (D-La.) and fellow committee members listen to testimony from Obama administration officials in the Dirksen Senate Office Building on Capitol Hill on July 10. (Chip Somodevilla/Getty Images)

Update 3:08 p.m.: This post has been updated to reflect the small sample size -- and thus, less-reliable results -- of the individual tiers.

A new study from a Democratic pollster shows part of the reason Republicans are now favored to win the Senate in November.

The Democracy Corps poll tested the top 12 Senate race in the country -- including 10 Democratic-held seats -- and found Republicans running ahead by two points. That's within the margin of error. A pretty close race.

The pollster, though, also broke the map down into three tiers:

Tier 1 (Democrats favored): New Hampshire, Colorado, North Carolina, Michigan

Tier 2 (the Democratic firewall): Alaska, Arkansas, Louisiana, Iowa

Tier 3 (GOP favored): Kentucky, Georgia, Montana, West Virginia

Courtesy: Democracy Corps

A major caveat: This survey didn't conduct full-size surveys in each state. It surveyed 1,000 likely voters across the 12 states. Thus, each tier by itself contains a small sample across multiple states with a higher margin of error -- not exactly ideal for drawing big, concrete conclusions. The numbers above should be taken with a grain of salt.

But we think these three tiers are somewhat instructive. The first tier are the races that Republicans could win if the election becomes a Republican wave. The third tier are the states that Republicans need to have -- including two of their own seats in Georgia and Kentucky. And the second tier is the Democratic firewall, including Sens. Mary Landrieu (D-La.), Mark Pryor (D-Ark.) and Mark Begich (D-Alaska) and an open seat in Iowa.

Republicans need to win six seats to take the Senate. They basically need to win seven of the eight races from Tiers 2 and 3.