DETROIT (AP) — Days before a pivotal primary, Mitt Romney is highlighting his economic plan in what was billed as a major campaign speech delivered at a cavernous and largely empty football stadium.Romney's speech repeated new tax proposals he released earlier this week, including lowering marginal individual tax rates by 20 percent. He offered few new policy ideas, though he did offer new details on how he would raise the retirement age to help keep Social Security solvent.The Detroit Economic Club hosted the speech at Ford Field, where the Detroit Lions play football. The crowd occupied just a portion of the field itself, and the tens of thousands of stadium seats sat empty.Romney is fighting an unexpectedly stiff challenge in his home state of Michigan from former Pennsylvania Sen. Rick Santorum, where voters go to the polls Tuesday.