Sen. John Walsh, D-Mont., leaves the Capitol in June 2014. Walsh will not seek reelection. (J. Scott Applewhite/AP)

The decision by Sen. John Walsh (D-Mont.) not to seek election in November in the wake of a plagiarism scandal is the latest piece of good news for Republicans as they strive to take control of the Senate in less than three months.

Walsh’s departure from the race came in the same week that two Republican senators — Pat Roberts in Kansas and Lamar Alexander in Tennessee — defeated tea party challengers in primary fights, ensuring that every GOP senator seeking reelection would be the party’s nominee.

These past seven days typified the fates of the two parties this election cycle. Democrats have been hit by retirements in tough states — Montana, West Virginia, South Dakota and, to a lesser extent, Iowa — and Republicans haven’t nominated the sort of extreme candidates who lack broader appeal in a general election.

Those realities — along with a national playing field in which a handful of incumbent Democrats are defending Republican-leaning seats in places where President Obama is deeply unpopular — have made a GOP takeover a better-than-50/50 proposition.

Let’s go through the races.

Walsh’s decision not to run takes what was an uphill climb for Democrats and turns it into something close to a no-chance race. (A committee of Democrats will pick the party’s nominee by Aug. 20.) Montana joins the contests for open seats in West Virginia and South Dakota in that category, meaning that, unless something drastically changes, Republicans should have three takeovers in the bank — a nice head start going into Election Day.

That means the party needs three more pickups to gain the Senate majority. And it has more than enough seats in play to do it. Democratic-held seats in Alaska, Arkansas, Colorado, Iowa, Louisiana and North Carolina are competitive at this point. (Races in Michigan, New Hampshire and Oregon seem to be moving in the Democrats’ direction.)

Of that group, the seats in Louisiana and Arkansas seem to be the most endangered for Democrats, in large part because of the strongly Republican nature of both states.

Sen. Mark Pryor (D-Ark.) has run a very good campaign, while Rep. Tom Cotton (R) has underwhelmed somewhat. (To be fair, Cotton, a freshman member of Congress, entered the race with impossibly high expectations.) And yet, the public polling in the contest gives Cotton a narrow edge. (Internal polling shows Pryor in a slightly stronger position.)

In Louisiana, Sen. Mary Landrieu (D) has a wide lead over Rep. Bill Cassidy (R) as well as two other Republicans in the contest. But Landrieu seems unlikely to win more than 50 percent of the vote Nov. 4, and if she doesn’t, she will face a runoff Dec. 6 against the second-place vote-getter, who is likely to be Cassidy. Head-to-head polling between Landrieu and Cassidy gives the slightest edge to the challenger.

Iowa, Colorado and North Carolina fit comfortably into the next tier of vulnerability. Iowa State Sen. Joni Ernst (R) has run a terrific campaign for the seat of retiring Sen. Tom Harkin (D) and has been aided by the stumbles of Rep. Bruce Braley (D). Republicans’ last-minute recruiting coup in Colorado landed them Rep. Cory Gardner, although Sen. Mark Udall (D) hasn’t been caught by surprise and is working hard to paint the GOP congressman as extreme on social issues. The North Carolina contest is the quietest close race in the country; Sen. Kay Hagan (D) isn’t well-defined as a candidate, but she has endured millions of dollars in spending by conservative groups relatively well. State House Speaker Thom Tillis performed well in the Republican primary, but his stewardship of the chamber will be a major issue this fall.

Then there is Sen. Mark Begich (D-Alaska), who has run a solid campaign and is well-known and liked in the state. Republicans have a late primary — on Aug. 19 — and former U.S. attorney Dan Sullivan is expected to emerge with the party’s nomination. Early polling gives Begich a slight lead, but Sullivan remains relatively unknown and would seem to have room to grow.

While Republicans are playing lots of offense this cycle, their path to the Senate majority is complicated by Democrats who are seriously contesting two GOP-held seats: in Georgia and Kentucky.

Of the two, Kentucky seems the better opportunity, given Sen. Mitch McConnell’s middling poll numbers and the able campaign being run by state Secretary of State Alison Lundergan Grimes. Polling gives McConnell a slight edge, but even his most ardent supporters acknowledge that his vote ceiling is somewhere between 51 and 52 percent.

Georgia’s open seat is competitive because of a terrific recruit by Democrats in Michelle Nunn. She has performed exceptionally well on both the fundraising and polling fronts. But Republicans picked businessman David Perdue as their nominee, countering Nunn’s “outsider” brand, and Georgia remains a comfortably Republican state — particularly in a midterm election such as this one.

Add it all up, and Republicans have enough races within the margin of error to think that even the slightest national breeze blowing in their favor — and that wind looks likely to be there — will be enough to push them over the top in a few of these very close contests.

Democrats’ best hope lies in localizing races in places such as Arkansas, Alaska and Colorado, and maybe picking off Georgia or Kentucky. That looks possible — but not probable — at the moment.