I don’t think they are easy. Russia spent tens of millions on the 2016 election and research found it had essentially no effect. The last three presidential elections were won by the candidate with the least money (and often by significant margins).

Elections are won by people voting for issues that affect them, not a rich boogeyman

I don't think the ease really matters when we're talking about nation-states who look at the outcome of US elections as having a major impact on their country's future. Ease matters for regular people. Nation-states are prepared to do whatever it takes, regardless of cost or manpower.

A good, obvious and well-documented example is that both British and German agents were spending millions to impact US elections in the WWII period. There are obviously today hundreds of lobbying groups dedicated to producing propaganda and swaying legislation in the US that benefits their country. Some of it is aboveboard, some of it is not.

Or through buying your candidate's rug pull coins.