LITTLE ROCK, Ark. (AP) — The chief backer of a proposal to raise Arkansas' severance tax on natural gas said Monday he will not be able to turn in thousands of signatures needed to put the measure on the ballot, ending his campaign for a tax hike that drew high-dollar and bipartisan opposition.Sheffield Nelson, a former natural gas executive, had been given until Monday to submit more petitions. He said he will not have the more than 40,000 additional signatures needed to qualify the measure for the November ballot.Election officials said last month that only 21,347 of the 69,774 signatures submitted by Nelson's group, the Committee for a Fair Severance Tax, were valid. At least 62,507 signatures from registered voters were needed."The final conclusion was you just couldn't (gather signatures) in that short a period of time," said Nelson, a former Republican candidate for governor.Nelson suspended his signature-gathering efforts last month, but had left open the possibility he would turn in more signatures.Currently, natural gas wells are taxed at between 1.25 percent and 5 percent of the value of the gas being extracted from the ground. Nelson's proposal would have increased the tax to 7 percent, which he said would help communities repair roads damaged by heavy trucks carrying drilling fluid.The proposal faced opposition from top Republicans and Democrats in the state. Arkansans for Jobs and Affordable Energy, a group funded primarily by natural gas firms, spent more than $1.7 million campaigning against the measure. The group applauded the defeat Monday."The fact Mr. Nelson fell so short of the signature requirement shows Arkansans were not supportive of a tax increase that would jeopardize thousands of Arkansas jobs and threaten a vital part of the state's economy," the group's chairman, Randy Zook, said. "I applaud the people of Arkansas for recognizing the harmful impact Mr. Nelson's severance tax measure would have had on our state."A prosecutor in east Arkansas has called for an investigation into possible fraudulent signatures for Nelson's measure and a casino legalization proposal. The high number of invalid signatures submitted has also prompted Arkansas lawmakers to review the state's petition process.Nelson said he would welcome any investigation into the invalid signatures."That is the only way you'll stop this thing from happening again," he said.___Andrew DeMillo can be reached at www.twitter.com/ademillo