Virtually all of the candidates who have expressed an opinion about the need to focus on infrastructure either have not outlined a funding solution, or focus on changing taxes on corporations.

Of those who have put forward at least some way to pay for boosted spending on roads, rail lines, airports and more, the ones who at least take a stab at a funding solution mostly align with the current congressional Democratic leadership, which has proposed funding come from rolling back the 2017 GOP tax code changes, or otherwise tweaking the tax code to wring more money out of wealthy corporations -- an approach that Republicans have already flatly rejected.

Amy Klobuchar was first out of the gate with an infrastructure proposal she put forward as a centerpiece of her campaign, calling for a $1 trillion investment ($650 billion of which would come from federal funds) in everything from traditional transportation infrastructure to green practices and clean water. Read more about her plan here.

In November, Joe Biden rolled out a revamped infrastructure plan with a focus reminiscent of many of former President Barack Obama's priorities: transit, high-speed rail, electric vehicle charging infrastructure, socioeconomic equity, job creation and a focus on reducing transportation's impact on the climate. Specifically, Biden calls for a $1.3 trillion investment over 10 years, with $50 billion in the first year going toward repairing roads and bridges, and another $10 billion over 10 years to building transit in high-poverty areas. He proposes paying for it by rolling back some of the 2017 GOP tax cuts, taxing the “super-wealthy and corporations,” ending fossil fuel subsidies and other changes.

Elizabeth Warren hasn't expressly called for a large boost in infrastructure investments, but she has said she wants to expand public transit and reduce the climate impact of various modes of transportation. She's also called for making all new vehicles electric by 2030 and requiring electric vehicle charging stations at every highway rest stop. She has said she's open to discussing raising the gasoline tax, which currently fuels most federal transportation programs, but worries it "falls hardest on working people … what I really want to see is, I want to see an ultra-millionaire's tax," she told CNN's Jake Tapper.

Pete Buttigieg came out late in the game with an infrastructure plan-- but while he lagged behind some other candidates, his plan was among the most detailed. Buttigieg's plan calls for doubling a Transpotation Department discretionary grant program to $2 billion per year, and proposes to prioritize the current backlog of needed road and bridge repairs, pledging to cut both in half by 2030. He also calls for "injecting" $165 billion into the Highway Trust Fund, which uses a tax on gasoline to pay for most federal infrastructure spending, while requiring DOT to propose a way to shift away from the gas tax to a fee based on how many miles someone traveled. Like some other candidates, he proposes to pay for this spending by overhauling the capital gains tax, repealing GOP tax cuts, and raising the estate tax.

Michael Bloomberg rolled out a proposal to spend $1.2 trillion over a decade on repairing existing roads and bridges, as well as a significant focus on expanding transit and walking and biking infrastructure. He also pledged to complete one high-speed rail corridor by 2025, with another to follow by 2030. This plan, along with other priorities, would be paid for through $5 trillion worth of adjustments to the tax code, including a wealth tax, changes to capital gains and the estate tax, raising corporate tax rates and more.

Beto O'Rourke's plan to invest in infrastructure comes as a piece of a 10-year, $1.5 trillion attempt to respond to climate change, much of which appears focused on research. Beyond that the specifics get nebulous, other than a line saying he wants some of it to go to grants that "cut commutes, crashes and carbon pollution," including increased access to transit options. Read more about his plan here.

John Hickenlooper has said he wants a $1 trillion infrastructure investment with a focus on clean energy and rural broadband.