MIAMI: The Libertarian Party, usually a political midget in the quadrennial battle between Democratic and Republican leviathans as they rumble toward the presidency, is picking its own champion this weekend at a time the choice may actually matter. With many American voters seeming dismayed, or at least uninspired, by the apparent choices on the major-party menu—Donald Trump or Hillary Clinton—Libertarians are hoping the nominee they select Sunday at their convention in Orlando, Florida, can lure large numbers of the disaffected. Gary Johnson, the former New Mexico governor who is expected to win the nomination, has made a larger than expected dent in recent opinion polls. At least two surveys put him in double digits in a hypothetical three-way race against the Republican Trump and the Democrat Clinton. That 10 percent figure might not hold up until November. Yet, with Clinton and Trump both having the highest negatives of any presidential candidates in modern times, the Libertarians feel energized.