In 1883, he was declared not duly elected on the grounds that he was a member of the Prince Edward Island Legislative Assembly at the time of the election, which violated the law prohibiting dual mandates. Shortly after winning his federal seat, he had submitted his resignation notice from the provincial legislature to two fellow MLAs, Malcolm McFadyen and Peter McLaren, in accordance with the process for resigning from the legislature, and the legislature had already held a by-election to replace him.[1] However, a procedural error by McFadyen and McLaren had left the resignation not properly registered,[1] thus leaving his provincial office technically double-seated and his eligibility for federal office in doubt. While several Canadian provinces did at the time allow people to simultaneously hold seats in both the provincial and federal legislatures, Prince Edward Island had enacted a law explicitly disallowing this.
The federal committee adjudicating the matter was dominated by the governing Conservative Party, whereas Robertson was a Liberal. The committee ruled that his resignation from the provincial legislature was not valid, and voted to unseat Robertson;[1] rather than holding a by-election, the committee directly awarded the seat to Augustine Macdonald, the Conservative candidate whom Robertson had defeated in the federal election.[1]
He was re-elected in 1887, but faced defeat again in 1891.
