(CNN) In the struggle between Republican Senate Majority Leader Mitch McConnell and the Democratic-controlled House, the immovable object may finally have met an irresistible force.

McConnell has been the immovable object: He's frustrated House Democrats by systematically blocking Senate votes so far on the lengthening list of bills they have passed, from gun control to additional protections for patients with preexisting health problems.

But McConnell's blockade faces a new challenge as the House turns to a series of bills meant to fight foreign interference in the 2020 election . Those measures, aimed at defending fundamental American institutions from foreign subversion, may be tougher for the Kentucky Republican to portray as partisan overreach than the bills the House has passed so far. And that could make them an irresistible force that strains his overall strategy of preventing action on any House legislation.

"It could be the thing that has the public home in on where the problem is, where the obstruction is," says Democratic Rep. John Sarbanes of Maryland, a leading author of the House election security agenda. "The public, and I understand that, they paint everything with a broad brush and they say Washington is dysfunctional. Here's a case study that they are going to be very interested in, that shows ... the problem is not with Washington, the problem is not broadly with Congress, the problem is with Mitch McConnell, who will not bring any of these things to the Senate floor."

McConnell's decision to methodically bar consideration of any of the House priorities already looms as a defining gamble in the GOP's effort to maintain its Senate majority in next year's election. He has leaned into his role as obstacle, portraying a Republican-controlled Senate as the last line of defense against a Democratic "socialist agenda" and calling himself the "Grim Reaper" for their legislative plans.

"I am indeed the 'Grim Reaper' when it comes to the socialist agenda that they have been ginning up over the House with overwhelming Democratic support, and sending it over to America," he declared in an interview on Fox News Channel last week. "Things that would turn us into a country we have never been." McConnell's campaign is even providing contributors with T-shirts featuring a tombstone for "socialism" on the front and a similar quote underscoring his determination to block the House agenda on the back.

The electoral impact of McConnell's strategy will likely be determined by which side successfully defines the agenda he is obstructing.

"He talks about that almost every opportunity he can," says Josh Holmes, McConnell's former chief of staff. "Being in opposition to health care plans that end private health insurance or environmental deals that basically shut down your electricity provider is something that he's pretty comfortable with."

But neither single-payer health care nor the Green New Deal, which Republicans are confident they can paint as unprecedented government intrusion into the economy, is likely to reach a vote on the House floor, much less pass the chamber, before 2020.

Instead, the legislation the House has passed this year -- and that McConnell is blocking -- has focused more on expressions of social values and bread-and-butter economic concerns, like buttressing the Affordable Care Act and confronting high prescription drug costs. Many of these measures enjoy preponderant support from the public in polls.

Popular legislation stalled

nearly three-fourths of Americans said they supported legal status for those young people. The House, for instance, in early June passed legislation that provides legal status for potentially millions of "Dreamers," young people brought to the country illegally as children. In a Fox News poll released Sunday,nearly three-fourths of Americans said they supported legal status for those young people.

dilute the Affordable Care Act's protections for patients with preexisting medical problems. In April Also in May, the House passed legislation to block regulatory actions by the Trump administration that woulddilute the Affordable Care Act's protections for patients with preexisting medical problems. In April polling by the nonpartisan Kaiser Family Foundation , about two-thirds of Americans said it was important that health insurers be required to sell coverage to consumers with preexisting health conditions and be prevented from charging them more.

Other House-passed measures this year include the Violence Against Women Act, legislation promoting greater gender equity in pay and comprehensive legislation to expand voting rights and impose new ethics guidelines on Washington. Senior Democratic House aides are confident that by the 2020 election, they will also pass legislation creating a nationwide $15 minimum wage, expanding the subsidies for families to purchase health insurance through the ACA's exchanges, updating the Voting Rights Act and combating the rising costs of prescription drugs.

The strong public support for most of these ideas has Democrats cautiously optimistic that their challengers next year can portray incumbent Republican senators as part of a "do-nothing Senate" blocking action on important concerns.

"When your occupation is to vote every day down the line against things that matter to voters ... sure, we are going to make those a significant issue," says J.B. Poersch, president of Senate Majority PAC, a leading Democratic super PAC.

Democrats have been frustrated so far by their inability to create more pressure on McConnell to take up any of the House-passed bills; House Speaker Nancy Pelosi held a news conference last week where she brandished a chart showing "McConnell's graveyard" of bills that he had blocked, complete with miniature tombstones.

Holmes, now president of a Washington communications firm, says House Democrats today face the same unforgiving equation Republicans did in 2013-14. During that congressional session, the GOP-controlled House passed a series of conservative priorities, only to see them systematically blocked by the Democratic majority led by then-Sen. Harry Reid of Nevada. Despite loud complaints, House Republicans could never generate enough pressure on Reid to force him to allow votes on the GOP plans.

McConnell feels politically secure bottling up the House priorities, Holmes says, "for the same reason Harry Reid didn't feel particularly moved by the fact that (then-Speaker) John Boehner had moved every conservative bill dealing with the economy or social issues possible. The priorities of Nancy Pelosi are not the priorities of Mitch McConnell, period. And he's very comfortable with that."

Election security measures

Election security, though, could be an issue that causes at least some GOP senators to question McConnell's blockade. Sarbanes, who chaired the House Democrats' Democracy Reform Task Force, says the party plans to pass by August "a suite" of bills to safeguard the 2020 election against foreign interference.

Those bills will include some measures already included in the House's omnibus political overhaul legislation, HR 1, that would provide states with more money to harden voting systems against possible foreign intrusion and mandate that the Department of Homeland Security develop a strategy for resisting such attacks.

The House also plans to pass new measures requiring campaigns to notify federal law enforcement officials if they are approached by foreign operatives with damaging information on their opponents, as well as provisions barring campaigns from sharing internal information with foreign officials, mandating more disclosure of foreign ad purchases on digital platforms and clarifying that it is illegal to work with foreigners to influence an American election.

"We may not get every single piece of this package onto the floor and passed and directed to the Senate before August but we want to get a good critical mass of these important measures in place," Sarbanes says.

Polls have shown broad public support for further action to resist interference by Russia or other foreign actors in the 2020 election. A Monmouth University poll last month found that 60% of Americans believe the government is not doing enough to guard against such interference; a survey by Democratic pollster Geoff Garin for the bipartisan advocacy group Law Works Action found that more than 4 in 5 Americans support a requirement that political campaigns notify law enforcement officials of foreign offers of assistance.

McConnell, as noted above, has felt comfortable blocking debate on other House-passed legislation with comparably lopsided levels of public support. But House Democrats are hopeful that more Senate Republicans will demand that he allow action on these issues because the public is likely to see them as less partisan.

"I think it's going to be a very difficult place for him to be, opposing these things that are supposed to protect the fundamental principles of our democracy," Sarbanes says. "This is about ... protecting ourselves from foreign interference, having confidence that our elections are being carried out in a free and fair and uninfluenced way. It's baseline stuff; it's Founding Fathers kind of principles here. If you stand in the way of measures that are designed to safeguard these principles, I think you are standing in the way of American democracy or at least not respecting it."

McConnell hasn't yet definitively closed the door on election security legislation. He's committed to a full Senate briefing later this month from intelligence officials on the possible risks.

"I would suspect that has a huge amount of influence about what ultimately the Senate does in this space," says Holmes.

But all signals from McConnell suggest he's unlikely to accept almost any new federal initiatives on election security. Last week, Senate Republicans blocked an effort by Democratic Sen. Mark Warner of Virginia to force a vote on a "duty to report" bill, which many Republicans consider an effort to embarrass President Donald Trump. McConnell shrugged off Trump's comments , which drew widespread condemnation in both parties, that "you might want to listen" to a foreign government offering dirt on a 2020 opponent. And Sen. Roy Blunt of Missouri, a member of the GOP leadership, has publicly declared that McConnell is unlikely to allow any election security bill to reach the floor, whatever the House does next.

That opposition reflects both McConnell's long-standing resistance to federal influence over any aspect of election activity (he's long been the leading opponent of campaign finance regulation) and his reluctance to take actions that provoke Trump. But it risks allowing Democrats to make election security a powerful symbol for the wider wall McConnell has constructed against the legislation passing the House.

In a sign of what may be ahead, the Texas Democratic Party on Monday, for instance, chastised Republican Sen. John Cornyn, who's up for reelection next year, for joining the Republican opposition to Warner's legislation on disclosing foreign contacts. "If Cornyn is unwilling to legislate, what exactly is he in Washington for?" Abhi Rahman, the party's communications director, said in a statement.

The ordinarily taciturn McConnell has enthusiastically accepted the persona of the "Grim Reaper" for House priorities. The question is whether the Senate leader is embracing that identity to a point that helps Democrats entomb the GOP Senate majority in 2020.