A last-minute Republican effort just six weeks before the Nov. 5 election to award all of Nebraska's electoral college votes on a winner-take-all basis appeared doomed after Monday, when a key Republican lawmaker said he opposed the proposal despite lobbying from Donald Trump's allies.

State Senator Mike McDonnell said in a statement that he would not support altering Nebraska's current system, which splits its electoral votes by congressional district.

"Nebraska voters, not politicians of either party, should have the final say on how we pick a president," McDonnell said.

Here's more on why the issue has come to the forefront in the 2024 presidential cycle.

Nebraska, Maine aren't winner-take-all

The U.S. does not elect presidents based on popular vote. If it did, Democratic presidential candidates would have won seven of the last eight elections, instead of the actual five times that has occurred.

A presidential candidate needs 270 of 538 electoral votes to win, and 48 states award their electoral votes on a winner-take-all basis. Maine and Nebraska are the exceptions.

WATCH l On how close 2020 was to a tie, and the 2024 pathways to victory: 

Why 0.008% of the U.S. population might determine the election | About That

Voters in seven swing states will determine the outcome of the U.S. presidential election in November. Andrew Chang breaks down each of the states in play for Kamala Harris and Donald Trump and their pathways to 270 electoral college votes.

Those who backed dropping the winner-take-all rule in Nebraska in the early 1990s argued that it would better reflect voters' views and ensure presidential candidates campaigned in the state.

Joe Biden won one of Nebraska's five electoral votes in the 2020 election, in the states's 2nd congressional district, while Donald Trump took one of Maine's four electoral college votes. But in a 306-232 overall electoral college win for Biden, those particular exceptions had no bearing on the outcome.

Omaha voters could make the difference

In an election expected to be among the closest in U.S. history, every electoral vote will count. 

With seven battleground states likely to determine the election as their voting preferences can swing either to Republicans or Democrats, Nebraska's 2nd district featuring Omaha looms large. It is the electoral college vote among the 10 in Nebraska and Maine that is most in doubt.

So, if Harris wins the three so-called "blue wall" battlegrounds of Wisconsin, Michigan and Pennsylvania while Trump wins the four Sun Belt battlegrounds of North Carolina, Georgia, Arizona and Nevada, the result would be a 269-268 split in the electoral college, with Harris ahead. CNN has reported that there are actually three plausible pathways to 269-269.

Trump's allies have spent months pushing Nebraska Republicans to consider changing the electoral system. U.S. Sen. Lindsey Graham, a South Carolina Republican who is close to Trump, met with Nebraska lawmakers last week to discuss the proposal.

"To my friends in Nebraska, that one electoral vote could be the difference between [Kamala] Harris being president and and not, and she's a disaster for Nebraska and the world," Graham said on Meet the Press on Sunday.

How does a tie help Trump?

If the electoral college is tied, the House of Representatives and Senate determine the occupants of the White House, per the Twelfth Amendment of the U.S. Constitution.

In the House, each state delegation in that scenario gets a single vote for president. It doesn't matter if it's California with its 52 House members, or Idaho, Hawaii and Montana with their two House reps. One state, one vote.

So, if there are 26 or more state delegations in the House which have a Republican majority after the election results, as was the case after 2020, it can be reasonably assumed that they would each vote for Trump.

The individual senators vote on the vice-presidential winner. The Democrats head into this election with the slimmest of margins in the Senate, but that could change after Nov. 5.

Theoretically, the two chambers could vote for a president and vice-president from opposing parties.

WATCH l Dave Wasserman of Cook Political Report on potential election scenarios:

What the path to the White House could look like for Trump or Harris

The race to the White House is heating up ahead of U.S. election night on Nov. 5. Chief political correspondent Rosemary Barton speaks with David Wasserman, elections analyst at the Cook Political Report, about the latest U.S. polls and potential election night scenarios. Plus, journalists Toluse Olorunnipa and Courtney Subramanian walk us through the latest news from the campaign trail.

Another brain-breaking scenario is if there are 25-25 ties in both chambers, in which case the Presidential Succession Act of 1947 states that, at least temporarily, the Speaker of the House becomes president.

Given the electoral college chaos of the 2000 election in Florida, which required a Supreme Court ruling, and the violence at the U.S. Capitol seen after the 2020 election based on false claims of electoral fraud, few observers want to see these tie scenarios tested.

Lone senator prevents Nebraska change

In order to change their system so close to the election, a two-thirds majority was required in Nebraska.

The Republicans have that number of legislators, but McDonnell was unmoved. Interestingly, McDonnell was elected as a Democrat but switched parties in April after the state party censured him for his anti-abortion stances.

McDonnell is not opposed to ever changing the system to winner-take-all, encouraging lawmakers to put the Electoral College question on the ballot in 2025.

For his troubles, McDonnell was blasted by Trump on Truth Social as a "grandstander" overlooking a "common sense, Republican" idea.