Mitt Romney’s selection of Wisconsin Rep. Paul Ryan as his vice presidential running mate is a decision that will impact congressional races this fall. Of that we are certain.

But, where might Ryan on the national ticket grow into a major issue? Based on demographic data, history and the candidates running, below are five races in which the Ryan ripple effect is worth watching in the fall.

Rep. Kathy Hochul (D-N.Y.) could face another race in which Medicare is a central issue. (Harry Scull Jr/AP)

* Montana Senate race: The GOP nominee, Rep. Denny Rehberg, voted against Ryan’s budget in the House not once but twice. It’s not hard to see why. Montana has a substantial elderly population, and Ryan’s proposal to revamp Medicare into a voucher system is grounds for a Democratic attack. Ryan’s proposal only affects those currently under 55. But national polling shows an aversion to changing Medicare at all. Pair that with Democratic attempts to cast Ryan and his supporters as enemies of seniors, and you have a potentially potent political weapon.

Exit polling in 2008 showed that voters over 65 years old represented 18 percent of the state’s electorate, a couple of ticks higher than the 16 percent national figure. Arizona Sen. John McCain won Montana seniors by four points, which was just half of his advantage nationwide among seniors.

On Saturday, Rehberg praised Ryan, but also offered a reminder that he hasn’t always agreed with the House Budget Committee chairman. The jury is still out on whether Democrats can leverage the Ryan VP pick as an effective way to tie Rehberg to the Wisconsin Republican, but the task is easier than it was on Friday.

* New York 27th District race: Now-Democratic Rep. Kathy Hochul unexpectedly won a three-way special election in 2011 in part because the issue of Medicare (and the Ryan plan) was a focal point of the campaign. Now, Hochul faces a tough reelection race in a Buffalo-area district in which a majority of voters preferred McCain in the 2008 presidential race.

One of the reasons Hochul won in 2011 was that her opponent, Republican Jane Corwin, was slow to respond to Democratic attacks tying her to a plan to “end Medicare.” Corwin even acknowledged during the campaign’s 11th hour that her response should have been quicker. She supported the Ryan plan, but did a poor job of explaining it and didn’t anticipate the issue would drown out other matters.

This cycle, Hochul faces Chris Collins, a Republican who has refused to take a position on the Ryan budget. He has pivoted to an attack tying Hochul to the Medicare Advantage cuts in President Obama’s health care law. Hochul has said she opposes the the cuts to Medicare Advantage, even as she supports the law more broadly.

The selection of Ryan as Romney’s running mate puts an issue Hochul deftly navigated in 2011 back into the spotlight. The Buffalo News piece (headlined “Collins is mum on Ryan budget”) was a front page story. Collins’s non-response could be an effective tactic, or it could end up leaving voters dissatisfied with him not taking a position.

* Florida Senate race: In April, Rep. Connie Mack IV, the likely Republican nominee against Sen. Bill Nelson (D), called the Ryan budget “a joke, doesn’t balance the budget for years.” His spokesman later said that Mack was calling the vote process, not the budget proposal itself, a joke. He added that Mack, who missed the spring vote, would have voted for the measure.

But the muddled response is ripe for Democrats to raise the issue again and ask where Mack stands. With Ryan on the GOP’s national ticket, and Florida retaining its status a highly-prized swing state at the presidential level, Romney’s campaign will be making a full court press in the fall, which means ample opportunity for Democrats to raise the issue down the ballot. Romney says he will run on his own budget, not Ryan’s. But that won’t stop Democrats from linking the two.

Seniors made up over a third of the 2010 electorate in Florida, according to exit poll data. As in Montana, arguments against Ryan could swing some senior voters over to the Democratic side, even though Ryan’s proposed policy does not affect them.

* Nevada Senate race: GOP Sen. Dean Heller is in a unique position: He voted for the Ryan budget in the House and Senate last year, but voted against it in the Senate this year. (Heller was appointed to fill the seat of Sen. John Ensign last year.)

Heller’s record on the matter could receive more scrutiny with Ryan on the national ticket. Democrats will be able highlight the discrepancy between his House vote and his Senate record. Seniors made up over 20 percent of the Nevada electorate in 2010, according to exit poll data, meaning that Heller must navigate the issue carefully with an influential portion of the electorate.

* North Dakota Senate race : While North Dakota is an increasingly red state, it’s not one that shuns government involvement. So an effort to reshape a long-standing entitlement isn’t going to play as well here as it would somewhere else. For Democrats, Rep. Rick Berg’s (R) votes in favor of the Ryan will fuel arguments that Berg’s views are out of step with the state.

Republicans have been wagering that their repeated attempts to tie Democratic nominee Heidi Heitkamp to the federal health care law will pay dividends, but that attack is more about tying her to Obama (who isn’t terribly popular in the state) than it is about connecting her to a government program. If Democrats can shift the focus from that story line to one about Ryan and Berg, then they will have dampened a potentially bruising line of attack.