Today Paul Ryan rolls out his new budget, which is expected to contain a new version of his Medicare plan. The proposal, which mirrors Mitt Romney’s plans, represents the GOP’s main bid in the broader argument that may decide the presidential and Congressional elections — and, ultimately, force the American people to make a big choice between two starkly different sets of priorities and ideological roadmaps for the country’s future.

Ed O’Keefe and Rosalind Helderman have the details:

* The plan would “slash trillions of dollars in federal spending, a move likely to appease conservative, tea-party-backed GOP lawmakers but infuriate the White House, congressional Democrats and some Republicans concerned that spending disagreements could spark threats of a government shutdown just weeks before the November elections.”

* “The proposal would replace the current tax structure’s six brackets with just two tax levels, a 10-percent marginal tax rate for lower income earners and 25-percent for upper income earners. That would be a reduction from a top marginal rate of 35 percent under the current structure. The plan would also lower the top corporate income tax rate to 25 percent.”

One key argument to watch: The Ryan budget is expected to reneg on the deal reached last summer to resolve the debt ceiling impasse, by setting discretionary spending lower than the agreed upon level. Republicans will likely justify this by arguing that the original compromise merely capped spending, which doesn’t preclude spending from being cut further.

But the White House and Dems will argue that both sides have previously described the deal as one setting levels of spending, rather than upper limits. Mitch McConnell recently acknowledged that what was actually agreed upon were “discretionary spending levels.” Eric Cantor also has described the deal as one that set a “level of spending.”

This side argument could be important in the coming spin wars.

* GOP strategists fret about Ryan Medicare proposal: The Medicare proposal is different from last year’s — it’s the plan agreed upon by Ryan and Dem Senator Ron Wyden — but GOP strategists are already worried about a rerun of last year’s political fiasco. Republican strategist Ron Bonjean:

“The verdict depends on if Republicans can quickly define this plan among seniors before the Democrats define it for them. Republicans had a turbulent practice run last year, so they should be well-prepared for the onslaught of negative attacks the other side will likely unleash.”

But is this really a matter of being prepared? Or is it that tampering with Medicare’s basic mission is politically toxic no matter how it’s presented?

* The optimistic GOP view of the politics: Jonathan Weisman sums up the Republican gamble: They believe “worries over the deficit will trump affection for Medicare and other popular programs,” and think voters will “reward them” for a “bold and realistic effort to transform and shrink government.”

* The Dem view of the politics: The DCCC has launched a “millionaires over Medicare” campaign against 41 House Repubulicans, seizing on the plan’s lower tax rates for top earners to frame the battle as one over fiscal priorities.And the labor-backed Americans United for Change has released an extensive video parody version of the rerun of the Medicare fight, this one featuring Romney’s full-throated endorsement of Ryan’s plan.

Dems and Obama’s outside allies hope to use this fight to make Medicare an albatross for the eventual GOP nominee, and it will be a key test of how badly the GOP primary has damaged Romney by forcing him to embrace positions that could be toxic in a general election.

* Questions remain about Ryan plan: Ezra Klein:

Ryan’s budget includes an ambitious tax reform proposal that takes the tax code down to two rates: 15% and 25%. But he’s not expected to say which income levels pay which rates. Moreover, he’s expected to say that the plan is revenue neutral because it will include closing tax loopholes, but he’s not expected to identify which tax loopholes will be closed. For obvious reasons, a tax reform plan that doesn’t explain who will pay which rate and how the lost revenue will be made up is not much of a tax reform plan, and it’s not an exercise in making tough choices. But perhaps Ryan will offer more detail than we think.

Needless to say, Ryan will be granted the presumption of fiscal seriousness whether or not he answers these basic questions.

* Ryan claims bipartisan support: Ryan’s op ed in the Wall Street Journal introducing his plan is larded with claims that his approach has bipartisan backing, something that — like the presumption of his fiscal seriousness — really deserves serious scrutiny from reporters.

* Romney prodded on birth control: A great campaign trail detail:

A female audience member asked Romney where he might suggest “millions of women go” without Planned Parenthood offering reproductive and sexual health care. “Well they can go wherever they’d like to go,” Romney told her. “This is a free society. But here’s what I’d say, which is the federal government should not tax these people to pay for Planned Parenthood.”

* Obama leading Romney in Virginia: A new Quinnipiac poll finds that Obama has opened up a 50-42 lead over Mitt Romney among registered voters in Virginia. He leads among women, 52-39, and among independents, 46-43. Virginia could potentially be key to reelection, because holding New South and/or western states could get Obama to 270 by offsetting expected losses in the Rust Belt.

* Big primary in Illinois today: If Romney wins today’s Illinois primary, a key test for him, it might have something to do with the fact that Romney and his Super PAC have outspent the Rick Santorum forces by seven to one. Romney continues to rely on an overwhelming financial advantage to enable him to triumph over weak competition.

* The war over the Violence Against Women Act: If you need to get caught up, Adam Serwer has a useful overview.

* And there’s just no end to Romney’s dissembling: Glenn Kessler demolishes Romney’s silly claim that “Obama’s regulators” banned Thomas Edison’s light bulb. It’s remarkable enough that this fantasy has had such staying power in certain precincts; it’s even more remarkable that it’s now been embraced by the likely GOP presidential nominee.

What else?