WASHINGTON — Congress is set to make it easier for political parties to collect bigger checks from their wealthiest backers — if Democrats don’t scuttle the plan first. Under a spending bill introduced in the Republican-led House, each donor would be allowed to give almost $1.6 million per election cycle to political parties and their campaign committees. The comparable limit for 2014’s elections was $194,400. The campaign finance proposal was tucked into an unrelated measure to keep most of …